It is known that a plastic lens having a high refractive index is obtained by a reaction between a polyisocyanate compound and a polythiol compound. For example, Patent Literature 1 discloses a method for manufacturing a polyurethane plastic lens having a high refractive index by heating a composition obtained by mixing an aliphatic polyisocyanate compound and an aliphatic polythiol compound such as pentaerythritol tetrakis-(thioglycolate) or trimethylolpropane tris-(thioglycolate).
Patent Literature 2 discloses a method of using a tetrafunctional polythiol compound such as pentaerythritol tetrakis(thioglycolate) or pentaerythritol tetrakis(mercaptopropionate) together with a bifunctional polythiol compound having a thiol group in order to increase the crosslinking degree of a resin.
In addition, an ester compound of mercaptopropionic acid or mercaptoglycolic acid and a polyhydric alcohol is widely used as a polythiol compound which is a raw material of a plastic lens. As for a raw material of the ester compound, some literatures disclose a correlation among quality of the raw material, impurities contained in the raw material, and quality of an obtained lens.
For example, Patent Literatures 3 to 6 disclose that the following problem occurs when the amount of impurities contained in pentaerythritol and mercaptocarboxylic acid as raw materials of pentaerythritol mercaptocarboxylate is increased. When the amount of impurities is increased, viscosity of a polymerizable composition obtained by mixing pentaerythritol mercaptocarboxylate and a polyiso(thio)cyanate compound is increased, and it may be difficult to handle the polymerizable composition. In addition, when the amount of impurities is increased, for example, a hue of a lens may be worse, or turbidity may be generated in a lens disadvantageously.
Therefore, a method for manufacturing a plastic lens described in Patent Literature 3 suppresses turbidity of an obtained lens by setting the content of bispentaerythritol in pentaerythritol to 5.0% by mass or less. In addition, a method for manufacturing a plastic lens described in Patent Literature 4 suppresses turbidity of an obtained lens by setting the total content of sodium and calcium in pentaerythritol to 1.0% by mass or less and by setting the content of bispentaerythritol to 5.0% by mass or less. Furthermore, a method for manufacturing a plastic lens described in Patent Literature 5 suppresses turbidity of an obtained lens by setting the content of a bimolecular condensation thioester in mercaptocarboxylic acid to a predetermined value or less. In addition, a method for manufacturing a plastic lens described in Patent Literature 6 suppresses turbidity of an obtained lens by setting the total content of an alkali metal and an alkaline earth metal in pentaerythritol to 1.0% by mass or less.